1. Field of the Invention
The present invention relates to a ferroelectric liquid crystal display device and more particularly, to a liquid crystal display device combining a switching device and a ferroelectric liquid crystal.
2. Description of the Relates Art
A liquid crystal display device is largely used in a watch, an electronic calculator, a word processor, a personal computer, a pocket television or the like. Recently, a display device of high quality which is capable of large-amount displaying has been especially demanded. As such a display device of high quality, a liquid crystal display device is generally known, which is formed by combining an active matrix substrate on which a thin film transistor (TFT) is arranged in the form of matrix with a twisted nematic (TN) liquid crystal.
However, a serious defect of the liquid crystal display device is that its visual angle is narrow. This problem is peculiar to the TN display. Therefore, as long as this display method is used, the defect can not be really improved. In addition, there is a strong demand for decreasing a driving voltage in respect of economical aspect.
Meanwhile, a visual angle is wide in a ferroelectric liquid crystal. Since the ferroelectric liquid crystal does not have a clear threshold value, if a pulse width for switching is increased, the driving voltage can be decreased theoretically. However, there is the following defect in the normal ferroelectric liquid crystal display device, high contrast is not likely to be obtained because of molecular fluctuation due to a bias voltage.
Thus, it is proposed that the ferroelectric liquid crystal is combined with the active matrix substrate in order to implement a liquid crystal display device in which the visual angle is wide, the driving voltage is low, and the contrast is high.
The ferroelectric liquid crystal display device (Appl. Phys. Lett., 36, 899 (1980); Japanese Opened Patent No. 107216/1981; U.S. Pat. No. 4,367,924) uses a ferroelectric liquid crystal such as a chiral smectic C phase, a chiral smectic F phase or a chiral smectic I phase. Although the ferroelectric liquid crystal has a herical structure, it is found that the helical structure is broken if the ferroelectric liquid crystal is sandwiched between liquid crystal cells having a cell thickness thinner than its herical pitch. Actually, as shown in FIG. 1, it is found that a region where a liquid crystal molecule is inclined from a normal line of a smectic layer by an angle of .theta. to be stable and a region where it is inclined in the opposite direction by an angle of .theta. to be stable can exist together. When an electric field is applied in the direction perpendicular to the surface of FIG. 1, the directions of the liquid crystal molecule and its spontaneous polarization can be uniformly arranged. Therefore, two states can be switched by changing the polarity of the electric field to be applied. Since bireferegent light is changed in the ferroelectric liquid crystal in the cell through the switching operation, transmitted light can be controlled by putting the ferroelectric liquid crystal display device between two polarizers. In addition, even if application of the voltage stops, since the orientation of the liquid crystal molecule is maintained at a state before the voltage is applied by orientation controlling force of an interface, a memory effect can be also obtained. In addition, in regard to a time required for driving the switching operation, a high-speed response of an order of .mu.sec can be obtained since the spontaneous polarization of the liquid crystal and the electric field directly act thereon.
FIG. 2 shows the relation between an applied voltage waveform in the ferroelectric liquid crystal display device in which the direction of the polarization axis of polarizing plates crossing at right angles with each other coincides with the direction of longitudinal axis of the molecule at one state of bistable states of the ferroelectric liquid crystal display device, and an amount of the transmitted light. Since the ferroelectric liquid crystal display device has a memory characteristic, preferable switching between two values can be implemented by applying a short pulse and maintaining an electroless state thereafter.
Next, the active matrix substrate will be described. FIG. 3 shows an equivalent circuit of an active matrix type liquid crystal display device using a thin film transistor (TFT) which is a typical 3-terminal type device.
Referring to FIG. 3, reference character G designates a gate electrode, reference character S designates a source electrode, reference character D designates a drain electrode, reference character V.sub.com designates a common electrode, and reference character LC designates a liquid crystal capacity. When the liquid crystal is driven, an electric field is applied to the gate electrode by applying a signal from a scanning line and then the TFT turns ON. In synchronization with this, when a signal is applied to the source electrode from a signal line, an electric charge is stored in the liquid crystal capacity through the drain electrode. Thus, the liquid crystal responds by the thus generated electric field.
In a case where the ferroelectric liquid crystal is employed to the active matrix type liquid crystal display, driving waveforms shown in FIGS. 4 and 5 are used.
However, according to the driving method shown in FIG. 4, for example, when display at a certain pixel is not changed for a long period of time, a voltage of the same polarity is applied to the ferroelectric liquid crystal of that pixel. This is a considerably big problem in view of reliability and it is almost impossible to manufacture a practical display device.
Meanwhile, according to the driving method shown in FIG. 5, the voltage applied to each pixel is not partial to minus nor plus, so that the method is thought preferable in view of reliability. However, the following problem is generated in view of a practical display device. That is, a pulse width required for switching the typical ferroelectric liquid crystal is approximately 100 .mu.sec at a room temperature when the voltage is 10V. Although a more high-speed ferroelectric liquid crystal has been reported, generally it is necessary to increase the spontaneous polarization of the ferroelectric liquid crystal in order to increase the switching speed of the display formed of a liquid crystal material. However, as the spontaneous polarization is increased, it is difficult to obtain preferable bistable switching operation. Since the driving voltage of the liquid crystal display device driven by the TFT is approximately 5V, when it is assumed that the driving voltage is 5V and the pulse width necessary for the switching is 200 .mu.sec, in the driving waveforms shown in FIG. 5, a writing time per scanning line is 600 .mu.sec. When the liquid crystal display device having 1000 scanning lines is implemented, a time required for rewriting one screen is 600 .mu.sec. If the driving voltage is decreased, that time is further increased. This is long as for the rewriting time. Therefore, this problem has to be improved in order to implement a large-capacity active matrix type ferroelectric liquid crystal display device of high quality.